2016 data now include culture-independent diagnostic testing results

Action Points

Note that this observational data from the CDC found that campylobacter and salmonella accounted for the most food-borne illness infections in the US in 2016.

But the increased use of culture-independent diagnostic tests has made the numbers from 2016 less easily interpretable with regards to earlier years.

Campylobacter and Salmonella were responsible for the highest number of reported bacterial foodborne-illnesses in 2016, CDC researchers said.

Overall, both pathogens each caused over 8,000 infections, confirmed either via traditional method or only by culture-independent diagnostic tests. Shigella was a distant third, involved in 2,913 bacterial foodborne-illnesses in 2016, according to Ellyn Marder, MPH, and colleagues.

But compared with 2013-2015, incidence of Campylobacter was significantly lower (11% decrease) when including only confirmed infections, but not significantly different when including both confirmed and those confirmed using only culture-independent diagnostic tests (CIDTs), the authors reported in Morbidity and Mortality Weekly Report.

By contrast, incidence of confirmed or CIDT positive-only Shiga toxin-producing Escherichia coli, or STEC, and Yersinia infections were both significantly higher in 2016, they noted.

But the authors explained that increased use of CIDTs by healthcare providers may be "complicating" these numbers. CIDTs are "quicker and easier to use" and may identify infections that would've remained undiagnosed. But unlike traditional culture methods, the authors said, the tests often do not yield bacterial isolates -- which public health officials rely on to "distinguish pathogen subtypes, determine antimicrobial resistance, monitor trends, and detect outbreaks."

Identifying these isolates via reflex culture (culturing a specimen with a positive culture-independent diagnostic test) then falls to either clinical laboratories or often state public health laboratories, which may not be able to absorb that burden as the cost of these tests continues to rise, they wrote.

The CDC cautioned that due to increased use of CIDTs, "comparisons of the 2016 data with data from the previous years may not accurately reflect trends."

"We need foodborne-illness trend data to monitor progress toward making our food supply safer," said Robert Tauxe, MD, director of the CDC's Division of Foodborne, Waterborne, and Environmental Illnesses. "It's important that laboratories continue to do follow-up cultures on CIDT-positive patients so public health officials can get the information needed to protect people from foodborne illness."

Researchers examined data from the CDC's Foodborne Diseases Active Network, or FoodNet. This is comprised of 10 U.S. sites in certain states and counties, which comprise 15% of the U.S. population (around 49 million people). A "confirmed" infection was defined as the detection of a parasite from a clinical specimen.

In 2016, FoodNet identified 24,029 infections, with 5,212 hospitalizations and 98 deaths from foodborne-related illness. FoodNet tracked nine pathogens and their changes in incidences in 2016 versus 2013-2015. The following were either confirmed or CIDT positive-only infections:

But almost a third of both Campylobacter and Yersinia and almost a quarter of STEC and Shigella infections were positive via culture-independent diagnostic testing, but without culture confirmation. Moreover, the increase in CIDT positive-only infections for six pathogens, including Vibrio and Salmonella, more than doubled versus 2013-2015 (114% increase, range: 85% to 1,432%).

The authors found that reflex culture was attempted on 60% of infections with a positive result via culture-independent diagnostic testing in either a clinical or state public health laboratory. These ranged from 45% for Campylobacter to over 85% for both STEC and Vibrio.

Examining reflex culture, the large majority of Salmonella and STEC infections were positive (88% and 87%, respectively), followed by Shigella (64%).

FoodNet also identified 62 cases of post-diarrheal hemolytic-uremic syndrome in children under 18 years of age (0.56 per 100,000), and 56% of those in children less than 5 years of age (1.18 per 100,000). However, there was no significant difference in incidence in either age range in 2015 versus 2012-2014.

The authors emphasized that these changes are important to consider when monitoring progress towards Healthy People 2020 goals, which were created before use of CIDTs and based on confirmed infections. They argued that both incidence measures and objectives might be reevaluated in light of these "changing diagnostics."